(1) Field of the Invention
This invention relates to testing and diagnostics of line processes used for the manufacture of semiconductor devices and more particularly to the measurement and monitoring of particulates in plasma etching and deposition tools.
(2) Description of Prior Art
The manufacture of large scale integrated circuits in a mass production manufacturing facility involves hundreds of discrete processing steps beginning with the introduction of blank semiconductor wafers at one end and recovering the completed chips at the other. In general the processing steps can be divided into two sections: The masterslice or front-end processing and the personalization or back-end processing. The former constitutes primarily of the high temperature processing steps wherein the discrete semiconductor devices are incorporated into to the surface of the wafers. These processes include diffusion, ion-implantation, oxidation, and epitaxial growth.
The personalization section involves lower temperature processing and comprises the formation of the interconnections of the devices by metal lines to form the various circuits. This processing is above the silicon plane and constitutes operations such as the deposition and etching of metal and insulator layers. In most instances at least two layers of metallization are required to form the hierarchy of circuits and provide pads for connection of the chip to its external package. Throughout the entire course of fabrication are photolithographic steps in which layers of masking materials are patterned and etched to create the desired design features.
The methods used to fabricate particular layers of material vary throughout the process. For example, at the beginning of processing a silicon oxide layer is usually formed by oxidation of the silicon wafer in a furnace at 900.degree. to 1100.degree. C. At a later point after impurities have been diffused into the silicon, such high temperature processing is no longer possible. Here, when a silicon oxide layer is required, low-pressure-chemical-vapor-deposition (LPCVD) may be used at temperatures between 550.degree. to 700.degree. C. After the first layer of metallization has been applied--this generally comprises an alloy of aluminum with small amounts of silicon, tungsten, or titanium--the allowable processing temperature yet further reduced. The layers can be deposited at temperatures below 400.degree. C. by plasma-enhanced-chemical-vapor-deposition (PECVD).
The machines or tools which perform PECVD can be used both for deposition and etching of insulating films such as silicon dioxide and silicon nitride. Unfortunately, films deposited in these plasma tools are not of the same quality as those deposited at higher temperatures. They tend to contain products of the reaction as impurities and are highly subject to particulate contamination. The particulates consist of deposits nucleated within the plasma, and debris flaking from the chamber walls, wafer platform, and other chamber components. Particulates are one of the most predominant chip yield detractors and their impact is felt at nearly every step of chip manufacturing. The sources of particulates and the degree of impact varies with each process step. In back-end processing the integrity of the metal lines and their interlevel via connections are compromised by particulates. Depending upon their occurrence, the particulates cause opens and shorts within the metal lines and when present in the insulating layers they can be responsible for interlevel shorts. Worse yet, they can produce weak spots in the metal lines or the insulating layers, which can impact chip reliability.
Low temperature deposition and etching tool such as those used for PECVD or plasma etching must be carefully maintained and closely monitored in order to minimize particulate contamination. Every effort is made in the design of the tool through shielding, cooling, and the like to prevent accumulation of deposits on the internal parts and the chambers walls. Nevertheless, such deposits build up as the tool is used. Frequent cleaning is required to keep the chamber fit. Usually, a normally running tool, does not need to be cleaned after each job. However, the period of cleaning cannot be defined by a fixed number of jobs. That is to say, that routine maintenance is not sufficient to assure a reliable tool. Such conditions as high room humidity or too rapid pressure changes within the system can aggravate particulate contamination. It thus becomes imperative to have a reliable monitoring system in place to give real-time information regarding the particulate count within the plasma tool.
Morioka et al. U.S. Pat. No. 5,274,434. have described a particle monitoring method adaptable to automated VLSI manufacturing lines which is capable of examining every product wafer. The heart of their invention is a unique optical foreign particle detection system. The wafer is scanned as it passes beneath a linear multi-lens array. Oblique light is provided by a corresponding array of semiconductor lasers positioned at an angle to the wafer. The reflected light passes through a set of spatial filters which contain the repetitive information of the product chips. This information can be filtered out of the data and only the non-repetitive features representing defects or particulates are noted.
The technique also provides for the detection of the orientation of the wafer by sensing either the wafer flat-edge or other alignment marks as may be used. Thus the coordinates can be adjusted to align with the pattern. The device with it's appropriate spatial filters and computer input circuitry can be placed at various stations along the production line, for example at the loading port of an automatic LPCVD deposition tool where wafers are fed into a transfer lock from a cassette. The lock is closed, evacuated, and the wafer or wafers are exchanged with completed wafers in the deposition chamber. The completed wafers are then scanned as they pass on their way back to the cassette. This monitoring device is most suited for well-established, single-product, automated mass production lines.
The technique of this invention is more process tool specific and is intended to monitor the cleanliness of a particular type of tool, regardless of product and thus is more applicable to smaller, multi-product, production lines as well as to development lines. Consequently, it is of a less complex nature and far less costly to implement.
The standard procedure for monitoring particulates in a single wafer plasma etch/deposit tool of the type treated by this invention is to place a clean bare or oxidized silicon wafer into the tool, evacuating the tool, turning on the gas flow for 30 to 60 minutes, removing the wafer and performing a particle scan using oblique light. This procedure is normally performed once each day or once for every 500 or so wafers and the particle counts are graphed. The standard procedure fails to give reliable particle count data because the action of a major particle generator is omitted during the monitor runs but does operate during production runs.
The plasma tool benefited by this invention is provided with a gas distribution housing located above and concentric with the wafer. The distribution housing is carefully designed to distribute the reactant and carrier gases over the wafer area so that their composition is maintained uniform over the entire wafer surface. Heating is provided to the housing and to the chamber walls to prevent the accumulation of reaction product polymers on these surfaces. This task is made difficult by the fact that compositional changes in the gas take place as reactants deplete and products are formed. Furthermore, because of viscous flow at the pressures used, effective gas mixing is not accomplished. In order to achieve better gas homogeneity during a process run, the distribution housing is set into an oscillatory motion. During conventional monitoring, however, a plasma is not struck and the gas distribution housing remains motionless. It has been found that the motion of the distribution housing during plasma processing is a major cause of particle generation. Tool abnormalities such as heater malfunctions can create temperature non-uniformities causing local polymer formation. The mechanical movement then causes particulates to dislodge from these sources.